The present invention relates to photodetectors. More specifically, the present invention relates to ultra-fast metal-semiconductor-metal (MSM) photodetectors.
Photodetectors are becoming increasingly important in modern electrical systems. These detectors are used in communication systems where information is transmitted in the form of light pulses along an optical fiber. In addition to the area of communications, such detectors are also used to read back optically stored information such as that stored on optical disks. Further, photodetectors can be used in high speed sampling, millimeter wave generation, remote sensing and generally to detect radiation. The speed of operation of the detector is crucial in determining the speed at which the detector can receive information.
Typically, photodetectors are fabricated using a material which generates charge carriers in response to incident radiation, such as from an optical communication signal. A plurality of interdigitated metal "fingers" are deposited on the material and connected to electrical contacts. A bias voltage is applied between the fingers. Current flow between the fingers is determined by the incident radiation. Current flow is detected and amplified.
These structures and their fabrication are described in "Nanoscale Tera-Hertz Metal-Semiconductor-Metal Photodetectors," by Stephen Y. Chou and Mark Y. Liu, published in the IEEE Journal of Quantum Electronics, Vol. 28, No. 10, October 1992.
Metal-semiconductor-metal (MSM) photodiodes are a family of fast, high-sensitivity detectors. Their simple planar structures enable easy fabrication in a process compatible with typical planar circuit technology. Hence, these structures are attractive for use in integrated opto-electronic systems. In the past, most attention was given to MSM diodes made on III-V substrates. Detectors fabricated in GaAs had speed, measured to the full width at half maximum (FWHM) of the response transient, as fast as 0.87 ps.
Although gallium arsenide-based photodetectors provide relatively fast response, it is preferable to fabricate devices in silicon. Silicon is a widely used semiconductor and easily implemented using present-day fabrication techniques. Further, typical silicon fabrication techniques tend to be less expensive than those of gallium arsenide. The Article, "32 HGz Metal-Semiconductor-metal Photodetectors on Crystalline Silicon," by Stephen Y. Chou, Yue Liu and T. F. Carruthers, published in Applied Physics Letters, 61 (15) on Oct. 12, 1992, by the American Institute of Physics, describes the fabrication of photodetectors in silicon.
However, preliminary results on 1.2 .mu.m and 300 nm silicon diodes measured with colliding pulse lasers (wavelength of about 620 nm) indicated much slower response (14 and 11 ps, respectively) than the III-V compound semiconductor detectors.
In addition, these silicon devices had a long "tail" response, a residual electron drift component past the main peak. The tail response was as long as 1.4 ns for the 1.2 .mu.m diodes. The tail is a particularly undesirable feature for practical applications. Because photoconductive current is cumulative, the device has to fully recover to its original, nonconductive state before the next optical pulse can be detected. This recovery period limits the speed of the device to less than 1 Gb/s. The speed of these devices became slower as the wavelengths of radiation become longer. At longer wavelengths, the devices lacked sufficient speed to be useful, particularly for applications such as telecommunications.
Thus, silicon photodetectors have not achieved the levels of performance which a simplified theory predicted.